In the border-collapse: collapse model, the width of a table must include half the table border according to CSS 2.1

1st col. & row 2nd col. thead 3rd col. 4th col. 5th col.
2nd row thead abcd abcd abcd
5th row tfoot a b c
3rd row tbody abc abc abc
4th row tbody ab ab ab

The relevant CSS code in that page is
body {margin: 15px 100px;} #idTable {border-collapse: collapse; border: lime ridge 100px; width: 500px;} td {blue solid 4px;}

Reference: CSS 2.1 Section 17.6.2 Border-collapse model says:
"Note that in this model, the width of the table includes half the table border"

The table width should correspond to the row width as described in this W3C CSS 2.1, Section 17.6.2 image

Firefox 3, Seamonkey 2.x all pass this test. Firefox and Konqueror 3.5.8 also pass this test but they return the wrong computed value for CSS width.

Valid HTML 4.01! CSS compliant